1. Industrial Field of the Invention
The present invention relates to a fuel pressure regulator used in a fuel injection system for an internal combustion engine.
2. Description of the Prior Art
Conventionally, a fuel injection system for an internal combustion engine includes a fuel pressure regulator for adjusting fuel pressure applied to fuel injection valves to be constant.
This fuel pressure regulator, as shown in FIG. 8, includes an air chamber 101 and a fuel chamber 102 which are separated from each other by a diaphragm 100. The fuel pressure regulator adjusts fuel pressure of the fuel chamber 102 (fuel pressure applied to fuel injection valves) to be higher than manifold negative pressure acting on the air chamber 101 by a predetermined degree (for example, 2.5 atmosphere).
When a difference between the fuel pressure and the manifold negative pressure becomes larger than the predetermined value, the diaphragm 100 is forced up to lift a movable valve 103 which moves in cooperation with the diaphragm 100. As a result, an opening degree of a throttle portion made up of the movable valve 103 and a valve seat 104 (a gap between the movable valve 103 and the valve seat 104) becomes large enough to return excessive fuel to a fuel tank (not shown), thereby maintaining the fuel pressure in the fuel chamber to be constant.
In the conventional fuel pressure regulator, when the fuel in the fuel chamber passes the throttle portion, the pressure of the fuel is abruptly decreased because the fuel is released from the high-pressure region (the fuel pressure applied to the fuel injection valves) to the atmospheric pressure circumstances, so that negative pressure or undershoot is apt to be generated at the throttle portion (a point A in FIG. 8).
In this way, the prior art has a disadvantage in that evaporation gas (evaporated fuel gas discharged into the atmosphere from a fuel system such as a fuel tank or a carburetor of an automobile) is liable to be generated owing to cavitation and deaeration caused by the occurrence of the negative pressure or the undershoot phenomenon.
At present, Evaporative Emission Control Law which restrains emission of the evaporation gas has been executed in the United States, because the evaporation gas causes air pollution. The Evaporative Emission Control Law will be further strengthened in the near future.